AN2836 datasheet(17/57 Pages) STMICROELECTRONICS

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AN2836

Designing an application with L6225, L6226, L6227

Doc ID 15084 Rev 1

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conditions. The 1N4148 diodes withstand about 200 mA DC (1 A peak), and the maximum

reverse voltage is 75 V, so they should fit for the majority of applications.

4.7

Sharing the charge pump circuitry

If more than one device is used in the application, it's possible to use the charge pump from

one L6225, L6226 or L6227 to supply the VBOOT pins of several ICs. The unused CP pins on

the slave devices are left unconnected, as shown in Figure 10. A 100 nF capacitor (C8)

should be connected to the VBOOT pin of each device. Supply voltage pins (VS) of the

devices sharing the charge pump must be connected together.

The higher the number of devices sharing the same charge pump, the lower the differential

voltage available for the gate drive (VBOOT - VS), causing a higher RDS(on) for the high-side

DMOS, thus higher dissipating power.

Better performance can also be obtained using a 33 nF capacitor for C5 and using Schottky

diodes (for example BAT47 are recommended).

Sharing the same charge pump circuitry for more than 3÷4 devices is not recommended,

since it reduces the VBOOT voltage increasing the high-side MOS on-resistance and thus

power dissipation.

Figure 10.

Sharing the charge pump circuitry

4.8

Reference voltage for PWM current control (L6227 only)

The L6227 has two analog inputs, VREFA and VREFB, connected to the internal sense

comparators, to control the peak value of the motor current through the integrated PWM

circuitry. In typical applications these pins are connected together, in order to obtain the

same current in the two motor windings. A fixed reference voltage can be easily obtained

through a resistive divider from an available 5 V voltage rail (maybe the one supplying the

µC or the rest of the application) and GND.

A very simple way to obtain a variable voltage without using a DAC is to low-pass filter a

PWM output of a µC (see Figure 11).

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